The present invention relates to boilers, and, in particular, to a boiler which includes an evaporator and an economizer.
In boilers of the type referred to above, water enters the economizer at a relatively low temperature and, in the economizer section of the boiler, is usually heated to just below the boiling point. Then, the hot water passes into the evaporator portion of the boiler, where it boils. The water and steam are separated in a drum, and the steam may then go on to a superheater, where it is heated to a temperature higher than its boiling temperature. The steam which leaves the boiler may then go to a turbine, where it performs work.
In the prior art, there have been many problems with these boilers. There is sometimes a problem with vaporization taking place in the economizer. In many cases, in order for the boiler to work most efficiently, the water which leaves the economizer must be close to the boiling point. However, if the water begins to boil in the economizer section, it can cause problems. The vapor can become trapped, causing vapor lock and water hammering, as well as fatigue, which can damage the boiler.
This problem occurs often under transient conditions. For example, if there is a need for a greater steam flow, the valve in the steam output line from the boiler is opened, reducing the pressure in the boiler. With the reduced pressure, more fluid boils in the evaporator. The rising volume of steam bubbles in the boiling water causes the water level in the drum to rise. If the water level goes too high, the steam quality is reduced, with some water entrained in the steam, and some water can enter the superheater and eventually damage it. Even if the steam does not go on to a superheater, the steam quality is important, and the water level in the drum must be maintained in order to maintain the steam quality. To prevent the water level from becoming too high, the water input to the boiler is reduced. With less water flow into the economizer, the water in the economizer is more likely to boil, creating the vapor lock, water hammer, and fatigue problems.
A common solution to this problem is to put a control valve or a small orifice in the line between the economizer and the evaporator, controlling the feed water supply, in order to raise the pressure in the economizer, making it more difficult for the water to boil. However, that means that the boiling takes place in the control valve or orifice instead, causing the valve or orifice to fail. It also means that more power is consumed, because the feed water pump must pump water across that large pressure drop, thus decreasing the efficiency of the power plant.
Another common solution is, once boiling begins in the economizer, to cause the feed water to bypass the economizer and go directly to the evaporator. This means that the economizer is not functioning for a good part of the time the boiler is operating, thereby greatly reducing the efficiency of the boiler. It also means that the economizer cycles between hot and cold as it goes from dry to wet, which causes wear and tear on the economizer.
U.S. Pat. No. 4,582,027 "Cuscino" shows a boiler in which the problem of boiling in the economizer is partially addressed. In this patent, a well-known bypass is provided, so that, under low load and start-up conditions, some of the fluid that has gone through the economizer does not go to the evaporator but is, instead, returned to the economizer. This keeps flow rates high enough to prevent boiling in the economizer. The teaching of this patent is intended to solve the problem of steaming in the economizer only during start-up and low load conditions, and for short periods of time--not during high flow rate conditions, where the boiler should be operating to be most efficient.